المؤشّران والنافذة المنزلقة

def has_pair(arr, target):
    left, right = 0, len(arr) - 1
    while left < right:
        s = arr[left] + arr[right]
        if s == target:
            return True
        elif s < target:
            left += 1
        else:
            right -= 1
    return False

def max_sum(arr, k):
    window = sum(arr[:k])
    best = window
    for i in range(k, len(arr)):
        window += arr[i] - arr[i - k]   # أضف الجديد واطرح القديم
        best = max(best, window)
    return best